There is also something with the frost plugs on the side of the block that denotes a two bolt or a four bolt block. I can't quite remember what it is, its been a lot of years since I,ve built any 400 sb stuff. I think the 4 bolt blocks have an extra frost plug on the sides of the block. Maybe someone else here knows? Also I would always prefer the two bolt over the factory 4-bolt blocks. The two bolts have more meat in the web area and when you put a steel splayed cap on they are very strong._________________Corey
CoMax Racing

Just an addition to what the other guys have posted.
Freeze plugs:
The 2 bolts generally have 2 freeze plugs with a blank cast in the block where the third one would be. The 4 bolts have three in the sides of them.
Cracks:
The 4 bolts tend to crack in the main web where the outer bolts of the 4 bolt tapped holes are. The 2 or 4 bolts can develope cracks in the deck from the steam holes to the head bolt holes. They can also crack in the deck where the large holes are. A screw in pipe plug with a small hole drilled in it can help strengthen the deck in this area.
Splayed Caps:
If you go with a 2 bolt block and use splayed caps which is recommended, during the installation you can potentially drill into water and have to be sealed.

-I have sonic tested and tested wall thicknesses by other means on a lot of 400's and there is two problems with the walls.
-The worst problem is: inside the water jackets below each headbolt hole is a notch (5 notches per cylinder) that runs the full length of the cylinder wall--top to bottom.
-These notches range from .129" to .101" on std bore blocks.
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-Wall thicknesses that I have found to be the best have always been on the 1969 to 1972 four bolt main blocks.
-The thickest walls I've seen are .238" on standard bores and the thinnest standard bore blocks I've seen .189"
-These are averaged numbers of the best and the worst blocks I've measured.
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-The worst blocks for wall thickness are casting numbers 330817.
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-If the engine is to be used for 400hp or so then it really doesn't matter much about whether it's a 2 or four bolt main and you can get away with a .030"-.035" overbore even on the thin wall blocks.
-I wouldn't ever consider more than a .035" overbore.
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-What happens is when you allow the engine to ping or rattle the vertical notches crack up and down one of those notches so you have to safely TUNE the engine solidly away from pinging that you can tolerate with a 350 block.
-The 350 bores can flex and spread the huge pressure spikes from the pinging because the walls aren't tied together in a siamese fashion and there are no notches to split the walls.
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-If you intend to try to make well over 400 hp say up to 600 on a normally aspirated engine you can do it but a two bolt main block with splayed caps, and a verified thickwall block, and serious attention to tuning away well from detonation.
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-400's have the external balanced cranks and that is bad news for anything over 5000rpm

-I'd prefer to run the bore at .005" over standard but those old engines are already bored .030" or too worn to bore and finish at much less than .025".
-I feel that 400 honest to GOD horsepower is about the limit without having the best of everything and having it near perfect.
-I have used some .040" stuff for stock real daily street driven pickup trucks with no problems but I would not personally waste any precious money on 400 with more than about a .025" overbore.
-When you get to around 600 real honest to GOD horsepower you can have a ton of money in the thing and I'd rather use a bowtie or a dart or something other than a stock 33 year old fragile 400 block.
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-You can move the bore centers by half the diameter you are boring the block---if the machinest is good but the head gasket fit to the bore diameter and the chamfer at the top of the block become a couple of the next issues.

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